(a) The organism in the environment
- Syllabus
- 2024
- Topic
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- Level
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Learning objectives
4.1Ecology termsUnderstand the terms population, community, habitat and ecosystem.4.2Comparing population sizePractical: investigate the population size of an organism in two different areas using quadrats.43B BiodiversityUnderstand the term biodiversity.44B Quadrat sampling practicalPractical: investigate the distribution of organisms in their habitats and measure biodiversity using quadrats.4.5Abiotic and biotic factorsUnderstand how abiotic and biotic factors affect the population size and distribution of organisms.
Build the ecological hierarchy
Ecologists describe organisms at nested scales, from one species in an area to all organisms interacting with the non-living environment.
| Term | Meaning | Example in one woodland |
|---|---|---|
| habitat | the place where an organism lives | the woodland floor |
| population | all organisms of one species in an area at a given time | all bluebells in the woodland |
| community | all the populations of different species in the area | bluebells, trees, fungi and animals |
| ecosystem | the community together with its abiotic environment and their interactions | the woodland community plus soil, water, light and temperature |
A community includes living populations only; an ecosystem adds the non-living environment. A habitat is a place, while a population is a group of one species living there.
Compare populations in two areas with quadrats
A fair quadrat investigation estimates the abundance of a plant or slow-moving organism in two areas using the same unbiased sampling method.
| Stage | Method and reason |
|---|---|
| 1 define areas | map each area and measure its total size |
| 2 sample randomly | use grid coordinates and a random-number generator to place equal-sized quadrats without choosing favourable patches |
| 3 measure | apply one counting rule to record individuals, frequency or percentage cover |
| 4 repeat | use many quadrats in each area to reduce the effect of patchiness |
| 5 compare | calculate abundance per quadrat or per m² and a mean for each area |
| 6 estimate population | mean density × total area, when a whole-area estimate is required |
Keep quadrat size, number of samples, species definition, counting rule, season and relevant time or weather conditions consistent between areas. Report variation as well as the mean when possible.
A quadrat samples a population; one quadrat is not the whole population. Convenience placement near paths or conspicuous organisms creates bias, and unequal effort makes the two areas incomparable.
Measure biodiversity as richness and evenness
Biodiversity is the variety of living organisms in an area. At species level it depends on both how many species are present and how evenly individuals are distributed among them.
| Component | Question it answers |
|---|---|
| species richness | How many different species are present? |
| relative abundance | What proportion of individuals belongs to each species? |
| evenness | Are individuals spread fairly evenly, or does one species dominate? |
| comparison | With equal richness, the community with greater evenness has greater biodiversity |
Two fields may each contain three plant species. A field with counts 15, 18 and 14 is more even—and therefore more diverse—than one with counts 19, 4 and 2, despite identical richness.
Counting species alone can be incomplete: two sites with the same richness may differ greatly in abundance and evenness. A large number of individuals of one species does not by itself mean high biodiversity.
Sample distribution and biodiversity with quadrats
Quadrats can reveal where organisms occur and allow biodiversity comparisons when placement is unbiased and sampling effort is sufficient.
| Investigation decision | Strong method |
|---|---|
| random distribution survey | overlay a coordinate grid; generate random x and y coordinates |
| change along a gradient | place quadrats at fixed intervals on a transect from the suspected factor |
| abundance measure | count individuals; for mats or lichens, estimate percentage cover with a gridded quadrat |
| biodiversity record | identify every species and record its abundance in every quadrat |
| reliability | repeat enough quadrats and compare means, spread, richness and evenness |
| valid comparison | keep quadrat area, sampling effort, identification rules, season and conditions consistent |
Density = total individuals counted ÷ total sampled area. Percentage cover = occupied grid area ÷ total quadrat area × 100. Distribution is supported by changes among locations, not just an overall total.
Random sampling tests a general area without observer choice; systematic transects test a spatial gradient. Neither means placing quadrats wherever organisms are easiest to see.
Explain population size and distribution with limiting factors
Population size and distribution reflect whether physical conditions and interactions with other organisms allow survival and reproduction at each location.
| Factor type | Examples | Route to population change |
|---|---|---|
| abiotic: non-living | light, temperature, water, oxygen, pH, salinity, mineral ions, wind | changes photosynthesis, respiration, enzyme activity, water balance or nutrient supply |
| biotic: living | food, competition, predation, disease, pathogens and mates | changes death rate, birth rate, access to resources or successful reproduction |
| distribution | a factor varies from place to place | organisms occur most where conditions fall within their tolerance and resources are accessible |
| population size | births and immigration versus deaths and emigration | size rises when gains exceed losses and falls when losses exceed gains |
Use data before proposing a mechanism: identify the population change, identify a factor that also changed, then explain the biological link. For example, more predators can increase prey deaths; fewer food plants can intensify competition and reduce reproduction.
Correlation suggests a possible factor but does not prove causation; several abiotic and biotic factors can change together. Biotic means involving living organisms, not simply any variable measured in biology.